In recent years, to improve the working efficiency of various industrial machines has been one of the important challenges. Thus, gears used in the industrial machines are required to be able to operate in a stable manner even in harsh conditions such as high speed and high load conditions. With an increase of required performance of gears, demands for high performance gear oils are also increasing. Thus, various additives for the oils have thus far been developed. For example, addition of an extreme pressure additive such as MoDTC is known to be effective for use in high load conditions and to be able to obtain an improvement of the transmission efficiency and an excellent resistance to abrasion.
However, MoDTC has a problem because of its poor heat resistance and its tendency to form a sludge and, therefore, is used only for a limited scope of application as an additive for gear oils. On the other hand, a gear oil composition which does not contain MoDTC and which has less tendency to form a sludge does not have fully satisfactory performance with respect to the transmission efficiency and wear resistance. Therefore, there still exists a demand for a further improved gear oil composition.
In this circumstance, there have been disclosed, in recent years, additives that aim at an improvement of various performances and gear oils containing such additives. For example, Patent Document 1 discloses a gear oil composition which is aimed at improving resistance to sludge formation and extreme pressure property and which contains a phosphorus-containing carboxylic acid compound and a phosphorothionate. Patent Document 2 discloses a gear oil composition which is aimed at improving resistance to sludge formation and water separating performance and which contains a dispersion-type viscosity index improver. Further, Patent Document 3 discloses a gear oil composition which is aimed at improving energy saving property and which contains a specific carboxylic acid or carboxylic acid derivative.
While Patent Documents 1 to 3 disclose gear oils which are to cope with energy saving of industrial machines, there is still a room for improving performances such as energy saving effect, resistance to sludge formation and extreme pressure property. Namely, there is a demand for a further improvement of performances.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2005-290181
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2005-290182
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2005-290183